We propose to study the joining of different DNA sequences--a phenomenon that we wish to call genetic fusion in Escherichia coli K12. This phenomenon is concerned with the formation of F- and R- prime factors, of transducing phages, and of mutations such as deletions, duplications, and translocations. Our previous work on the structure of F-prime factors suggests that there are certain sites and certain DNA segments which are particularly active in genetic fusion--and that the formation of F-prime factors may not be merely examples of illegitimate recombination with no site specificity. These studies will be carried out by genetic methods and by electron microscope heteroduplex analysis on F- and R-primes, on their segregants, and on transducing phages. These studies may provide new understanding of the general characteristics of genetic fusion, including duplication phenomena, by identifying some of the special characteristics of the DNA sequences involved.